A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a “mask” or a “reticle,” may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called “steppers,” in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called “scanners,” in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (e.g., the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate such as, for example, an overlay error between successive layers formed in or on the substrate. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This may be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometers are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (e.g., intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly-resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle. Polarized radiation beams may be used to generate more than one spectrum from the same substrate. Each library entry contains data representative of a pupil image which is, in turn, dependent on several metrology tool specific hardware parameters. These parameters include an angle of beam incidence, numerical aperture, wavelength range, polarization, illumination conformity, and noise. These parameters may vary between items of metrology hardware, even for the same type of scatterometer. Furthermore, these parameters may (for a single piece of metrology hardware) display a time variation due to wear-induced drifting of the metrology hardware parameters.
Thus, it is desirable for the library to contain parameters specific to the metrologies at all times. It is also desirable that the library include parameters relating to a profile of a grating that under measurement; that is, the library should contain information on the parameters relating to the sample with associated material parameters. If the calculation time of a library is, for example, about 30 minutes, then for 10 scatterometers of different parameters, 5 hours of calculation time is required.
U.S. Pat. No. 6,721,691 (the '691 patent), which is incorporated by reference herein in its entirety, discloses a method and system for incorporating the effects of small metrology hardware and material-based parameter variations in a library of simulated diffraction spectra. In particular, the '691 patent discloses a method for modifying the library diffraction spectra so as to be optimized for the particularly parameters of a specific piece of metrology hardware and specific samples. A parameter modification vector, which describes the differences between actual measurement parameters and parameters used in the calculation of the library spectra, is determined and used to calculate a corresponding modification to each library diffraction spectrum.